1. Field of the Invention
The present invention relates to an equivalent Wollaston prism for an optical pick-up head and an optical pick-up head using the same, and more particularly to a Wollaston prism for an optical pick-up head that is compact in size and cheap, and an optical pick-up head using the equivalent Wollaston prism which is easy to align.
2. Description of Prior Art
Ordinarily, an optical pick-up head for a magneto-optical disk system essentially consists of a collimating lens, several beam splitters, and other lenses, so as to detect a RF signal, focusing signal, and tracking signal. These signals are then conducted onward for further processing whereby data can be extracted from an optical disk. Usually the pick-up head is constructed of more than ten components which are difficult to assemble and align. Consequently, Wollaston prisms, as disclosed in U.S. Pat. Nos. 4,771,414 and 4,951,274, are used to solve the problem of complexity. Taking U.S. Pat. No. 4, 771,414 414 as an example, the structure of a pick-up head using the Wollaston prism is shown in FIG. 1. Pick-up head 10 essentially consists of a semiconductor laser 11, a collimating lens 12, a first polarizing beam splitter 13, an objective lens 14, a second polarizing beam splitter 15, a first light receiving lens 16, a first photodetector 17, a phase compensator 18, a half-wave plate 19, a Wollaston prism 20, a second light receiving lens 2, and a second photodetector 22. A laser light beam L is emitted from the semiconductor laser 11, then collimated by the collimating lens 12. The first polarizing beam splitter 13 deflects the laser light beam L toward the objective lens 14, then the laser light beam L is focused on an optical disk 30. The reflected laser beam L is focused by the objective lens 14 and then deflected by the first polarizing beam splitter 13 toward the second splitter 15. Laser beam L is split into a first laser beam L1 and a second laser beam L2. The first laser beam L1 is directed to the photodetector 17 through the first light-receiving lens 16. The photodetector 17 receives the laser beam L1 and generates a detection output signal. An error signal generating section 31 receives the detection output signal of the photodetector 17 and produces a focusing error signal Sf and a tracking error signal St for the focusing servo-control and the tracking servo-control respectively. The second laser beam L2 passes through the half-wave plate 19 straight to the Wollaston prism 20 through a phase compensator 18. The second laser beam L2 is further divided into a P-polarized component LP and a S-polarized component LS, then focused on the second photodetector 22 through the second light receiving lens 22. An information signal generating section 32 receives both the P-polarized component LP and the S-polarized component LS, then generates a reproduced information signal Si.
However, in the conventional optical pick-up apparatus above discussed, the information signal Si, the focusing error signal Sf, and the tracking error signal St are detected separately, therefore creating a bulky complicated, and expensive structure for this apparatus. Furthermore, the Wollaston prism in this apparatus, which is composed of two dual refractive prisms, requires a great deal of space, is very expensive, and is difficult to align. Most of all, this kind of Wollaston prism guides the laser beam directly, so the entire system must be arranged in a straight line, therefore limiting how much the structure can be minimized.